German Patent 35 31 632 (and also U.S. Pat. No. 4 953 524 which refers to said German Patent) teaches a radial piston engine comprising a housing, an annular cam disk fixedly mounted in said housing, a cylinder block rotatably mounted about an axis and with respect to said cam disk, a plurality of bores arranged in said cylinder block radially with respect to said axis of rotation, a plurality of pistons respectively mounted in said bores, and cylindrical rollers for supporting said pistons on said cam disk, said rollers having axes which extend parallel to the axis of rotation of the cylinder block, and wherein said bores of said cylinder block which are adapted to receive said pistons are provided with recesses in the area of movement of said rollers, so as to receive sections of said rollers on both end faces of said rollers. Said recesses comprise circumferential grooves which need to have a certain depth so as to allow an insertion of said rollers into said bores provided in said pistons, while at the same time, a lateral guidance of the rollers is provided at rest or contacting surfaces of the lateral wall of the cylinder block. So as to achieve this goal, a relatively large depth is required for the circumferential grooves. This leads to a weakening of the segments remaining between each two adjacent piston bores, i.e. segments which are necessary to support the pistons. Said segments are subjected to bending forces due to the effect of the piston force during operation. Thus the danger of breakage exists. This is particularly so for continuous operation of the engine.
German Offenlegungsschrift 38 28 131.7 (to which the above U.S. patent corresponds), published after the priority date of the present application, discloses a radial piston engine which intends to further develop in particular the radial piston engine of German Offenlegungsschrift 35 31 632 such that the disadvantages of such known engines of this type are avoided. In particular, the dangers of a breakage or breakage during long term operation should be excluded.